The usual method of forming active (high-surface area) carbon from polymers is by anerobic pyrolysis. It has been found, however, that the object being carbonized shrinks during the process. This means that a desired shape cannot be carbonized with retention of configuration. The final carbon has very high specific surface area, but the surface is not easily accessible; a relatively slow diffusion through the pores is the only way for the absorbed species to be captured. Thus, visualizing a massive block of such carbon, there would be no free path for the passage of a gas or liquid current. For this reason, charcoal, e.g., when used as a filter for cigarettes, is used in granular form, where the gas travels through the interstices between granules and is adsorbed by diffusion laterally into the granule.
The most effective configuration would appear to be an open-cell carbon foam. Such foam can be made by pyrolysis of phenolic foams or balsa wood, but due to the extremely fine cells in the original material, are resistant to flow. If a polymer could be foamed to the proper cell structure, then carbonized, and ideal balance might be arrived at between specific surface area and low back-pressure. However, thermoplastic polymers do not retain the desired configuration during pyrolysis and thermoset polymers have undesirable cell structure.
It is also known to make active carbon by pyrolysis of vinylidene chloride polymers. However, such polymer decomposes while melting, liberating HCl gas and turning to a bubbling tar before becoming carbonized.
It is further known that vinylidene chloride polymers can be carbonized by reaction with strong bases, such as alkali metal amides in liquid ammonia solution. Such process is not desirable, however, as such strong bases leave a residue (metal chloride) in the pores of the foamed structure which is non-volatile at temperatures less than about 1500.degree.C.
The present invention overcomes such disadvantage by utilization of specific foamed structures and liquid ammonia as the sole reactant wherein the only interstitial residue formed is NH.sub.4 Cl which, in turn, may be easily removed by moderate heating to its decomposition temperature (at 350.degree.C.).